An electro-pneumatic control system for the temperature control of motor vehicles is known, for example, from the British Pat. No. 1,205,946. The apparatus described there includes a pneumatic actuator which engages a servo valve. The compressed air for actuating the servo motor is generated by an air pump that is controlled via an electro-pneumatic relay. The pressure prevailing in the actuator is sensed by a bellows whose signal is a feedback signal for the electro-pneumatic relay permitting a comparison by means of a system of levers referred to magnetic force which depends on a sensor signal. The system of levers actuates an electro-pneumatic contact which turns the air pump on and off or opens and closes the air pressure system.
In the known apparatus, the air pump operates only intermittently, which has a detrimental effect on the control pressure which in turn causes disturbances in the overall control behavior. Furthermore, it is possible to excite the servomotor and the actuator into a state of oscillation.
Another apparatus which uses a temperature-dependent electronic control is known from the British Pat. No. 1,397,426. In this known apparatus, solenoid valves are controlled by relays or transistors to control the generally available vehicle vacuum. However, in this and similar systems, in which use is made of the prevailing engine vacuum which is modulated to engage a servo-actuator, two serious disadvantages arise. Firstly, the manifold pressure of the engine varies within the operational domain of the engine, causing erratic behavior of the pneumatic control system which uses the vacuum. Secondly, the use of the engine vacuum for control purposes tends to have deleterious effects on other vehicle systems, especially the carburetor if present.
In still other cases, especially in Diesel engine equipped vehicles, the available vacuum may be insufficient for any control purpose.